I have a component which requires children (called layout component). There is also a function inside this component that iterates over children and does something. I want to test this function. So I've extracted this function to separate utility class and give children as parameter. It works well in browser.
But in test I have problem. I don't know how to define and pass children to this utility function.
const children = (
<div />
<div />
);
gives me compilation error: "JSX expressions must have one parent element.ts(2657)".
Surrounding divs with React.Fragment
const children = (
<>
<div />
<div />
</>
);
gives we me only 1 child in my layout component.
I'd like to pass children to the utility function but I don;t know how to define them.
Instead of :
const children = (
<div />
<div />
);
You can use a real array e.g.:
const children = ([
<div key="0"/>,
<div key="1"/>
]);
Or just use the fragment as you have already figured out.
Related
Ok so here is a simplification of my real-world use-case. Basically I have a complicated layout and I pass the layout component to the child so that it renders itself within it and then the parent layout displays it as its children. Works great, except I use this in a loop and I cannot figure out how to set the key on the children from the parent.
It's strange because I can set other properties without any issues. In the example below, the "id" property is correctly set from the list, but the console emits the warning:
Warning: Each child in a list should have a unique "key" prop.
const Child = Parent => (
<Parent>
The child is reponsible
<br />
for writing the contents.
</Parent>
);
const present = (title) => {
const Tag = ({ children }) => (
<article id={title} key={title}>
<h1>{title}</h1>
{children}
</article>
);
return Child(Tag);
};
const app = (
<div>
{[
'hello',
'this',
'is',
'a list'
].map(present)}
</div>
);
ReactDOM.render(
app,
document.getElementById('root')
);
here's a demo of that code on codepen
Is there any way to accomplish what I'm trying to do?
This may seem like an over-complicated way to render a template, I know, but I have - I think - a good real-world use-case.
Actually I figured it out.
I need to use a React.Fragment around the return value of "present",
like so:
const present = (title) => {
const Tag = ({ children }) => (
<article id={title}>
<h1>{title}</h1>
{children}
</article>
);
return (
<React.Fragment key={title}>
{Child(Tag)}
</React.Fragment>
);
};
the problem is you are providing the key to the child & not the parent of what is being rendered from the map items.
you can set key to the parent component (instead of <article>) as well as sending the Tag parameter:
const Child = (Parent, key) => (
<Parent key={key}>
The child is reponsible
<br />
for writing the contents.
</Parent>
);
//...
const present = (title) => {
const Tag = ({ children }) => (
<article id={title}>
<h1>{title}</h1>
{children}
</article>
);
return Child(Tag, title);
};
your solution by using a React.Fragment also does a similar job (setting the key for the outer wrapper (here being fragment) in rendering the list using the map method). the difference being you are creating a fragment for each iteration in the map method, but by passing the key you can avoid that as well.
I got a little carried away and wrote
<Foo
style={ }
...
/>
in the old days, we actually had Foo as a <div> usually, so that could make sense, to actually impose that style on the <div>. But later on, we actually had <> ... </> and it'd be not good to have ReactJS impose an element (such as <div>) containing all those elements, so the style line doesn't make sense in this case.
So in a way, we never could treat a component as an element and give it some basic HTML attributes? How would we set HTML attribute for a component?
We could pass all HTML attributes to a React component as a prop and then inside the component's render function assign those props to the actual DOM HTML element.
const Foo = ({style}) => (
<div style={style}> // assigning inline style to HTML dom element
This is Foo
</div>
)
<Foo style={color: 'red'} /> // passing style as a prop to Foo react component
You can use JSX spread attributes and make it more flexible so you won't have to specify individual HTML attributes as a prop. You simply spread restProps over your HTML element.
const Foo({ name, ...restProps }) => {
return <div {...restProps}>{name}</div>;
}
Now you can pass DOM attributes to Foo and they'll be passed through to div.
<Foo
name="This is Foo"
className="foo-class"
id="my-foo"
style="color: red"
/>
When you use <> ... </> this is a shorthand of <React.Fragment>...</React.Fragment> which lets you only group multiple list of children WITHOUT adding extra node to the DOM when it gets rendered.
return (
<>
<Foo />
<Foo />
<Foo />
</>
);
This will render as
<div>This is Foo</div>
<div>This is Foo</div>
<div>This is Foo</div>
so if you find yourself need to pass a HTML attribute to <>...</> you can simply change react fragment to a <div>...</div> or <span>...</span> or other proper HTML elements and set your attribute on.
I want to do something like this.
In the parent component
<child tag={h1}/>
In the child component
<this.props.tag />
The problem "Unresolved variable or type div" throwing When i pass one of html components ( tags ) like ( div , h1, ext. )
UPDATED:
Yes, we can pass HTML tag as a prop. There are several ways based on what you want.
Passing tag as a prop
<ChildComponent tag="h1" />
And inside child component, we can use the tag as below.
const Child = ({ tag: Tag}) => (
<Tag>Hello World</Tag>
)
By setting dangerouslySetInnerHTML
<Child tags="<h1>Hello world</h1>" />
Inside child component:
const Child = props => <div dangerouslySetInnerHTML={{ __html: props.tags.outerHTML }}/>
Here is what you should know about dangerouslySetInnerHTML. In short, this exposes XSS attack.
This one is not related to passing as a prop. But you might wanna consider
If you are doing SEO task related (maybe nextjs) and you need to render conditional tag (sometimes h2 and sometimes h3). Then you can do as follow!
Conditional Statement
// Parent
const Parent = () => <Child isH3Tag />
// Child
const Child = ({ isH3Tag = false, children }) => isH3Tag ? <h3>{children}</h3> : <h2>{children}</h2>;
Here is a demo. https://codesandbox.io/s/8x65l707yj
JSX expressions must start with a cap, for example <Elment />
In your case, when you want to pass tag names, use createElement:
<div>
{React.createElement(this.props.tag, {children: <content>, prop1: <v1>, ...})}
<div>
Another alternative will be to use recompose#componentFromProp
You can pass tag as children, from parent Component like this:
In the parent component:
<Child> <h1></h1> </Child>
In the child component you can access like:
render() {
return (
{this.props.children}
)
}
I'm rather new to react.js and I wonder if it's possible to render a collection of different components with React?
For example I have a ContainerComponent class with a property childComponents which I want to use like this:
var childComponents = [];
childComponents.push(<TextComponent text={this.props.text} />);
childComponents.push(<PictureComponent src={this.props.image} />);
return (
<ContainerComponent
title={this.props.title}
innerComponents={childComponents} />
)
So the problem is that I can't call child components' render function in loop to generate single output of all children. How could render method of ContainerComponent look like?
Thanks!
Place all the components like you would normally do:
render() {
return (
<div>
{this.props.innerComponents}
</div>
);
}
Normally, the following code should work fine for your needs:
<ContainerComponent
title={this.props.title}
>
<TextComponent text={this.props.text} />
<PictureComponent text={this.props.image} />
</ContainerComponent>
I am still a noob at React and in many examples on the internet, I see this variation in rendering child elements which I find confusing. Normally I see this:
class Users extends React.Component {
render() {
return (
<div>
<h2>Users</h2>
{this.props.children}
</div>
)
}
}
But then I see an example like this:
<ReactCSSTransitionGroup
component="div"
transitionName="example"
transitionEnterTimeout={500}
transitionLeaveTimeout={500}
>
{React.cloneElement(this.props.children, {
key: this.props.location.pathname
})}
</ReactCSSTransitionGroup>
Now I understand the api but the docs don't exactly make clear when I should be using it.
So what does one do which the other can't? Could someone explain this to me with better examples?
props.children isn't the actual children; It is the descriptor of the children. So you don't have actually anything to change; you can't change any props, or edit any functionality; you can only read from it. If you need to make any modifications you have to create new elements using React.CloneElement.
https://egghead.io/lessons/react-use-react-cloneelement-to-extend-functionality-of-children-components
An example:
main render function of a component such as App.js:
render() {
return(
<Paragraph>
<Sentence>First</Sentence>
<Sentence>Second</Sentence>
<Sentence>Third</Sentence>
</Paragraph>
)
}
now let's say you need to add an onClick to each child of Paragraph; so in your Paragraph.js you can do:
render() {
return (
<div>
{React.Children.map(this.props.children, child => {
return React.cloneElement(child, {
onClick: this.props.onClick })
})}
</div>
)
}
then simply you can do this:
render() {
return(
<Paragraph onClick={this.onClick}>
<Sentence>First</Sentence>
<Sentence>Second</Sentence>
<Sentence>Third</Sentence>
</Paragraph>
)
}
Note: the React.Children.map function will only see the top level elements, it does not see any of the things that those elements render; meaning that you are providing the direct props to children (here the <Sentence /> elements). If you need the props to be passed down further, let's say you will have a <div></div> inside one of the <Sentence /> elements that wants to use the onClick prop then in that case you can use the Context API to do it. Make the Paragraph the provider and the Sentence elements as consumer.
Edit:
Look at Vennesa's answer instead, which is a better explanation.
Original:
First of all, the React.cloneElement example only works if your child is a single React element.
For almost everything {this.props.children} is the one you want.
Cloning is useful in some more advanced scenarios, where a parent sends in an element and the child component needs to change some props on that element or add things like ref for accessing the actual DOM element.
In the example above, the parent which gives the child does not know about the key requirement for the component, therefore it creates a copy of the element it is given and adds a key based on some unique identifier in the object. For more info on what key does: https://facebook.github.io/react/docs/multiple-components.html
In fact, React.cloneElement is not strictly associated with this.props.children.
It's useful whenever you need to clone react elements(PropTypes.element) to add/override props, without wanting the parent to have knowledge about those component internals(e.g, attaching event handlers or assigning key/ref attributes).
Also react elements are immutable.
React.cloneElement( element, [props], [...children] ) is almost equivalent to:
<element.type {...element.props} {...props}>{children}</element.type>
However, the children prop in React is especially used for containment (aka composition), pairing with React.Children API and React.cloneElement, component that uses props.children can handle more logic(e.g., state transitions, events, DOM measurements etc) internally while yielding the rendering part to wherever it's used, React Router <switch/> or compound component <select/> are some great examples.
One last thing that worth mentioning is that react elements are not restricted to props.children.
function SplitPane(props) {
return (
<div className="SplitPane">
<div className="SplitPane-left">
{props.left}
</div>
<div className="SplitPane-right">
{props.right}
</div>
</div>
);
}
function App() {
return (
<SplitPane
left={
<Contacts />
}
right={
<Chat />
} />
);
}
They can be whatever props that makes sense, the key was to define a good contract for the component, so that the consumers of it can be decoupled from the underlying implementation details, regardless whether it's using React.Children, React.cloneElement, or even React.createContext.